TB
Meaning of TB: Infection and disease are not the same thing here, and most people who carry the bacteria never become ill or infectious.
Definition of TB
TB is the everyday abbreviation for tuberculosis, an infection caused by the bacterium Mycobacterium tuberculosis. It spreads through the air when someone with active disease in the lungs or airways coughs, sings or speaks, releasing particles small enough to stay suspended. The lungs are the usual site, but the organism can settle in lymph nodes, bone, kidney or the lining of the brain.
What separates latent infection from active disease?
| Feature | Latent infection | Active disease |
|---|---|---|
| Symptoms | None | Cough lasting weeks, fever, night sweats, weight loss |
| Contagious | No | Yes, when the lungs or airways are involved |
| Chest imaging | Usually normal | Usually abnormal |
| Bacteria in sputum | Absent | Often present |
Once the bacteria are inhaled, the immune system normally walls them off inside a knot of cells called a granuloma. The organisms are contained but not always killed, and they can sit there for decades. Most people in that state never become ill. The balance tips when immunity weakens, which is why untreated HIV infection, immune suppressing medicines, poorly controlled diabetes, malnutrition and advanced age all raise the chance that contained infection becomes an opportunistic infection with active disease.
How is it found?
A skin test and a blood test both show that the immune system has met the organism, but neither can say whether disease is active, and both may stay positive for life. Answering that question means looking for the bacteria themselves. Chest imaging shows the damage, and sputum is examined under the microscope, grown in culture and tested by molecular methods that identify the organism and several resistance genes within hours. Culture remains the reference standard, though the organism grows so slowly that results take weeks. Disease outside the lungs is harder to reach and often needs a tissue sample from the affected site.
Why does treatment take so long?
The organism divides slowly, shelters inside the very cells sent to destroy it, and sits within walled-off tissue that drugs penetrate poorly. Clearing it therefore takes several drugs together, over months rather than days. The combination exists because any large population contains a few naturally resistant organisms, and a single drug simply clears the field for them. Interrupted or incomplete courses are strongly associated with relapse and with multidrug resistant strains, which need longer treatment using drugs that work less well and are harder to tolerate.
Also known as
Used in a sentence
The patient was started on a multidrug antibiotic regimen after testing confirmed active TB.
Good to know
Latent TB infection, in which the bacteria are present but inactive and the person is not contagious, is distinct from active TB disease, which produces symptoms and can spread to others.